HENNINGS, PETER H., and JON E. OLSON
The geometry and intensity of fractures in the Cretaceous Frontier Sandstone of the hydrocarbon-productive Oil Mountain Anticline (OMA) are, in part, related to the deformation path of the parent structure. OMA is the southern culmination of a line of anticlines produced by southwest-dipping backthrusts emanating from the leading edge of the Casper Arch thrust. OMA results from folding in advance of a thrust that steps up-section from a layer-parallel detachment at the base of the Ordovician Bighorn Fm, to its tip within the Chugwater Fm. Additional layer-parallel shear, distributed throughout the detached portion of the anticline, produced overturning of the forelimb.
A NW striking set of regional joints can be observed in nearby flat-lying beds of Frontier Fm. Younger systematic joint sets are observed only in folded beds and roughly parallel the direction of Laramide shortening. Joints occur in domains of varying intensity. Some joint swarms developed into tear faults, with the older joints accommodating shear and the newer joints forming in a rotated stress field.
We correlate the geometry and evolution of joints to a 3D kinematic model of bed curvature generated by volumetric restoration. Where the gently-dipping limb of the fold merges into the plunging nose, the sandstone beds accommodated the curvature by an evolution of joints into small faults. The faults separate large blocks which remained intact. At the nose of the fold where curvature is highest, the Frontier Formation. deformed by pervasive shearing. Along the steep limb, jointing and minor faulting are again dominant.